plagiarism.org defines plagiarism as follows:
"Many people think of plagiarism as copying another's work, or borrowing someone else's original ideas, but terms like "copying" and "borrowing" can disguise the seriousness of the offense"
According to the Merriam-Webster Online Dictionary, to "plagiarize" means
In other words, plagiarism is an act of fraud. It involves both stealing someone else's work and lying about it afterward."
You may hear that things that are "common knowledge" need not be documented or cited, but what is "common knowledge," and how do you know if something is common knowledge? Generally speaking it is a fact that is fully documented in many places and is known by many people. However, simply being on Wikipedia or any other Internet site does not make something common knowledge.
Facts that are not common knowledge or your personal interpretation or paraphrasing of ideas must be documented.
"It is normal and common practice to use the ideas of others for academic research and study. New discoveries and ideas are usually based upon the research findings, observations or ideas of other people (researchers, scientists, scholars, etc.). This is how scholarship is produced and perpetuated. When one uses the ideas of another, one must 'give credit where credit is due' and that is back to the authors or originators of those ideas. By not acknowledging the authors or originators of the ideas you are using, especially when writing papers or addressing a forum, you are committing plagiarism.
Here are some examples of plagiarism:
Ke, Irene and Loretta Wallace. "Avoiding Plagiarism" guide. University of Houston Libraries
This tutorial, available from LIONTV: Library Information Literacy Online Network, provides a good overview of plagiarism and how to avoid it.